Since it is desirable that the amount of steel sheets used for food cans and beverage cans is decreased in view of decreasing environmental load and cost, the thickness of steel sheets has been reduced. However, with the reduction in thickness of steel sheets, following problems have been exposed. That is, can bodies become deformed by external forces which are applied when cans are handled in can making, in content filling, in transportation, and in the market. Further, can bodies become deformed (buckled) by external forces which are applied to the cans due to the increase and decrease of pressure inside and outside the can when, for example, a heat sterilization treatment is performed on the contents of the can.
As a countermeasure for these problems, steel sheets have been strengthened in order to increase the strength of can bodies. However, the increase in the strength of a steel sheet decreases the shape fixability when roll forming is performed in order to form a can into a cylindrical shape before a seam welding is performed, and an appropriate width is not achieved for an overlapping portion of the steel sheet when welding is performed. Accordingly, in the case of a three-piece can where the can body is welded by performing seam welding, troubles occur in the welding process.
In addition, an increase in the strength of a steel sheet is accompanied by a decrease in ductility at the same time, and decreases formability for, for example, neck forming and flange forming which are performed on a can body after the welding.
In addition, in many cases of three-piece cans which are used as beverage containers such as coffee cans, cans are formed by roll forming in a direction at an angle of 90° to the rolling direction of a steel sheet, that is, in the width direction of a steel sheet. Regarding the mechanical properties in the width direction of a steel sheet, in general, since the strength is higher than that in the rolling direction and ductility is lower than that in the rolling direction, it is difficult to apply a strengthened steel sheet to such kinds of cans.
As described above, the strengthening of a steel sheet is not necessarily an optimum method for compensating for a decrease in deformation resistance due to the reduction in thickness of a steel sheet.
In the first place, the buckling of a can body occurs due to a decrease in the rigidity of a can caused by reduction in thickness of the can body. Therefore, it is considered that, for increasing buckling resistance, it is effective to increase Young's modulus (longitudinal elasticity modulus) of a steel sheet and thereby improve the rigidity of the can body. There is a strong correlation between Young's modulus and a crystal orientation. It is known that, in the case where there is a large amount of crystal orientation group (α fiber) having the <110> orientation parallel to the rolling direction, there is an increase in Young's modulus in a direction at an angle of 90° to the rolling direction. As examples of steel sheets for cans which have been developed in order to increase Young's modulus, the following techniques are disclosed.
Patent Literature 1 discloses a technique for manufacturing a steel sheet for a container having an increased Young's modulus in a direction at an angle of 90° to the rolling direction, in which a strong α fiber is formed by performing second cold rolling with a rolling reduction of more than 50% after performing annealing of a cold-rolled steel sheet.
Patent Literature 2 discloses a technique for manufacturing a steel sheet for a container having an increased Young's modulus in a direction at an angle of 90° to the rolling direction, in which a strong α fiber is formed by performing cold rolling with a rolling reduction of 60% or more on a hot-rolled steel sheet and performing no annealing.
Patent Literature 3 discloses a technique for manufacturing a steel sheet for a container having an increased Young's modulus in a direction at an angle of 90° to the rolling direction. Ti, Nb, Zr, and B are added to ultralow-carbon steel. Hot rolling is performed with a rolling reduction of at least 50% or more at a temperature equal to or lower than the Ar3 transformation point, and annealing is performed at a temperature of 400° C. or higher and equal to or lower than the recrystallization temperature after the cold rolling.
Nowadays, on the other hand, there is a case where a steel sheet is formed into a characteristic shape in order to give a design effect to a can by additionally giving elongation strain in the circumferential direction to the can after the steel sheet has been formed into a cylindrical shape and welded. A can which is formed in such a manner is called an unusual-shaped can. Since there is an increase in the rigidity of a can body due to the effect of the shape of such an unusual-shaped can, the strength of the can body increases. In particular, this strengthening is effective with respect to buckling caused by the increase and decrease of pressure inside and outside the can when, for example, a heat sterilization treatment is performed on the contents of the can. A steel sheet which is used for such an unusual-shaped can is required to have sufficient ductility for preventing fracturing from occurring when being formed. In addition, it is necessary that the yield point elongation of a steel sheet which is used for an unusual-shaped can should be controlled to be low in order to prevent stretcher strain from occurring. In addition, it is necessary to prevent an increase in the grain size of a steel sheet which is used for an unusual-shaped can in order to prevent surface deterioration from occurring. Moreover, it is necessary that the Lankford value (r value) of a steel sheet which is used for an unusual-shaped can is low in order to prevent the height of the can from decreasing.
In particular, in many cases of three-piece cans which are used as beverage containers such as coffee cans, welding is performed so that a direction at an angle of 90° to the rolling direction of a steel sheet, that is, the width direction of the steel sheet, is the circumferential direction of the can body. In this case, tensile deformation occurs in the circumferential direction of the can body when the can body is formed into the can body of an unusual-shaped can. Due to the tensile elongation in the circumferential direction, compressive deformation contrarily occurs in the height direction of the can. As a result, the can height decreases. It is effective to decrease an r value in the circumferential direction for suppressing such a decrease in the can height. As examples of techniques related to steel sheets having such a property, the following techniques are disclosed.
Patent Literature 4 discloses a manufacturing method including heating a steel material containing, by mass %, C: more than 0.05% and 0.1% or less, Mn: 0.3% to 1.5%, Al: 0.01% to 0.1%, B: 0.0002% to 0.01%, and N: 0.0030% or less at a heating temperature of 1050° C. to 1300° C. Finish rolling is performed on the heated steel material with a finish rolling temperature of 800° C. to 1000° C., coiling the hot-rolled steel sheet at a coiling temperature of 500° C. to 750° C., thereafter performing pickling and subsequent cold rolling. Continuous annealing is performed on the cold-rolled steel sheet at a temperature equal to or higher than the recrystallization temperature and 720° C. or lower, and second cold rolling is performed on the annealed steel sheet with a rolling reduction of more than 8% to 10%. This is a technique in which an r value is decreased and aging character is improved by appropriately controlling the contents of Mn and B in particular among the constituents of steel.
Patent Literature 5 discloses a technique, in which at least one of the r values in the rolling direction and a direction at a right angle to the rolling direction is 1.0 or less. Hot rolling is performed on a steel slab containing C: 0.0005 to 0.05 wt % and B: 0.0002 to 0.01 wt % with a finish rolling temperature of 800° C. to 1000° C. and the hot-rolled steel sheet is coiled at a coiling temperature of 500° C. to 750° C. First cold rolling is performed, annealed by soaking in a temperature range from the recrystallization temperature to 850° C. for a soaking time of 60 seconds or less, and subsequently second cold rolling is performed with a rolling reduction of 20% or less. This is a technique in which a decrease in can height due to forming is suppressed by determining an r value.